The broad objective is to understand the role and importance of uracil-DNA glycosylase in DNA repair. This enzyme removes uracil from both A-U and G-U basepairs; the former will result if dUTP is incorporated into DNA instead of dTTP, while the latter will be produced if cytosine in DNA is deaminated in situ. Both kinds of basepairs might affect the functioning of the DNA, but only cytosine deamination should be very mutagenic. By use of T5 dut and thy mutants, and E. coli dut, thy, and ung mutants, DNA containing various amounts of uracil can be synthesized in vivo. The biological function of this uracil-containing DNA is being studied.